Exhibit 5.1

[LETTERHEAD OF SKADDEN, ARPS, SLATE, MEAGHER & FLOM LLP]

March 10, 2014

McKesson Corporation

One Post Street

San Francisco, California 94104

Re: McKesson Corporation

Registration Statement on Form S-3

Ladies and Gentlemen:

We have acted as special counsel to McKesson Corporation, a Delaware corporation (the “Company”), in connection with the public offering of $400,000,000 aggregate principal amount of the Company’s Floating Rate Notes due September 10, 2015, $700,000,000 aggregate principal amount of the Company’s 1.292% Notes due March 10, 2017, $1,100,000,000 aggregate principal amount of the Company’s 2.284% Notes due March 15, 2019, $1,100,000,000 aggregate principal amount of the Company’s 3.796% Notes due March 15, 2024 and $800,000,000 aggregate principal amount of the Company’s 4.883% Notes due March 15, 2044 (collectively, the “Securities”), issuable under the Indenture, dated as of December 4, 2012 (the “Indenture”) between the Company and Wells Fargo Bank, National Association, as trustee (the “Trustee”). On March 5, 2014 the Company entered into an Underwriting Agreement (the “Underwriting Agreement”), with Goldman, Sachs & Co. and Merrill Lynch, Pierce, Fenner & Smith Incorporated, as representatives of the several underwriters named therein (the “Underwriters”), relating to the sale by the Company to the Underwriters of the Securities.

This opinion is being furnished in accordance with the requirements of Item 601(b)(5) of Regulation S-K under the Securities Act of 1933, as amended (the “Securities Act”).

McKesson Corporation

March 10, 2014

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In rendering the opinions set forth herein, we have examined and relied on the following:

(i) the registration statement on Form S-3 (File No. 333-179879) of the Company relating to the Securities and other securities of the Company filed with the Securities and Exchange Commission (the “Commission”) on March 2, 2012 under the Securities Act allowing for delayed offerings pursuant to Rule 415 of the General Rules and Regulations under the Securities Act (the “Rules and Regulations”), including information deemed to be a part of the registration statement pursuant to Rule 430B of the Rules and Regulations (such registration statement, being hereinafter referred to as the “Registration Statement”);

(ii) executed copies of the global certificates evidencing the Securities (the “Note Certificates”);

(iii) an executed copy of the Indenture;

(iv) an executed copy of the Underwriting Agreement;

(v) an executed copy of a certificate of Willie C. Bogan, Associate General Counsel and Secretary of the Company, dated the date hereof (the “Secretary’s Certificate”);

(vi) a copy of the Amended and Restated Certificate of Incorporation of the Company, certified by the Secretary of State of the State of Delaware as of February 26, 2014, and certified pursuant to the Secretary’s Certificate;

(vii) a copy of the Amended and Restated By-Laws of the Company, as in effect as of the date hereof, certified pursuant to the Secretary’s Certificate; and

(viii) copies of certain resolutions of the Board of Directors of the Company, or the appropriate committee of the Board of Directors, adopted on January 24, 2012, October 26, 2012 and January 28, 2014, certified pursuant to the Secretary’s Certificate.

We have also examined originals or copies, certified or otherwise identified to our satisfaction, of such records of the Company and such agreements, certificates and receipts of public officials, certificates of officers or other representatives of the Company and others, and such other documents as we have deemed necessary or appropriate as a basis for the opinions set forth below.

McKesson Corporation

March 10, 2014

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In our examination, we have assumed the genuineness of all signatures, including endorsements, the legal capacity and competency of all natural persons, the authenticity of all documents submitted to us as originals, the conformity to original documents of all documents submitted to us as facsimile, electronic, certified or photostatic copies, and the authenticity of the originals of such copies. As to any facts relevant to the opinions stated herein that we did not independently establish or verify, we have relied upon statements and representations of officers and other representatives of the Company and others and of public officials.

Our opinions set forth herein are limited to the General Corporation Law of the State of Delaware (the “DGCL”) and the laws of the State of New York that, in our experience, are applicable to transactions of the type covered by the Underwriting Agreement, the Indenture and the Securities and, to the extent that judicial or regulatory orders or decrees or consents, approvals, licenses, authorizations, validations, filings, recordings or registrations with governmental authorities are relevant, to those required under such laws (all of the foregoing being referred to as “Opined on Law”). We do not express any opinion with respect to the law of any jurisdiction other than Opined on Law or as to the effect of any such non-Opined on Law on the opinions herein which laws are subject to change with possible retroactive effect.

The Indenture and the Note Certificates are referred to herein collectively as the “Transaction Agreements.”

Based upon the foregoing and subject to the limitations, qualifications, exceptions and assumptions set forth herein, we are of the opinion that the Securities have been duly authorized by all requisite corporate action on the part of the Company and duly executed by the Company under the DGCL, and when duly authenticated by the Trustee and issued and delivered by the Company against payment therefor in accordance with the terms of the Underwriting Agreement and the Indenture, the Securities will constitute valid and binding obligations of the Company entitled to the benefits of the Indenture and enforceable against the Company in accordance with their terms under the laws of the State of New York.

The opinions stated herein are subject to the following qualifications:

(a) the opinions stated herein are limited by applicable bankruptcy, insolvency, reorganization, moratorium, fraudulent transfer, preference and other similar laws affecting creditors’ rights generally, and by general principles of equity (regardless of whether enforcement is sought in equity or at law);

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March 10, 2014

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(b) except to the extent expressly stated in the opinions contained herein, we do not express any opinion with respect to the effect on the opinions stated herein of (i) the compliance or non-compliance of any party to any of the Transaction Agreements with any laws, rules or regulations applicable to such party or (ii) the legal status or legal capacity of any such party to any of the Transaction Agreements;

(c) except to the extent expressly stated in the opinions contained herein, we have assumed that each of the Transaction Agreements constitutes the valid and binding obligation of each party to such Transaction Agreement, enforceable against such party in accordance with its terms;

(d) we do not express any opinion with respect to the enforceability of any provision contained in any Transaction Agreement relating to any indemnification, contribution, exculpation or waiver that may be contrary to public policy or violative of federal or state securities laws, rules or regulations; and

(e) to the extent that any opinion relates to the enforceability of the choice of New York law and choice of New York forum provisions contained in any Transaction Agreement, the opinions stated herein are subject to the qualification that such enforceability may be subject to, in each case, (i) the exceptions and limitations in New York General Obligations Law sections 5-1401 and 5-1402, and (ii) principles of comity or constitutionality.

In addition, in rendering the foregoing opinions, we have assumed that neither the execution and delivery by the Company of the Transaction Agreements nor the consummation by the Company of the transactions contemplated thereby, including the issuance and sale of the Securities: (i) constitutes or will constitute a violation of, or a default under, any lease, indenture, instrument or other agreement to which the Company or its property is subject, (ii) contravenes or will contravene any order or decree of any governmental authority to which the Company or any of its property is subject, (iii) violates or will violate any law, rule or regulation to which the Company or its property is subject or (iv) requires or will require the consent, approval, licensing or authorization of, or any filing, recording or registration with, any governmental authority under any law, rule or regulation of any jurisdiction.

We hereby consent to the filing of this opinion with the Commission as an exhibit to the Company’s Current Report on Form 8-K, being filed on the date hereof, and incorporated by reference into the Registration Statement. We hereby consent to the reference to our firm under the caption “Legal Matters” in the prospectus supplement dated March 5, 2014 and filed with the Commission. In giving this consent, we do not thereby admit that we are included in the category of persons whose consent is required under Section 7 of the Securities Act or the Rules and Regulations. This opinion is expressed as of

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March 10, 2014

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the date hereof unless otherwise expressly stated, and we disclaim any undertaking to advise you of any subsequent changes in the facts stated or assumed herein or of any subsequent changes in applicable law.

Very truly yours,

 /s/ Skadden, Arps, Slate, Meagher & Flom LLP